The present invention relates to alloys of synthetic resins and more particularly to alloys of modified styrenic resins with polyamides and to a method for the preparation of alloys of styrenic resins with polyamides.
Aliphatic and aromatic polyamides, widely termed nylon resins, are generally incompatible or at best are only poorly compatible with hydrocarbon polymers such as polystyrene and the copolymers and graft polymers of styrene with acrylic esters, acrylonitriles and the like. Blends of certain nylon resins with some styrenic resins are known in the art, such as for example the blends of epsilon caprolactam polymers with acrylonitrile-styrene-butadiene (ABS) graft polymers disclosed in U.S. Pat. No. 3,134,746. Even though these known prior art blends exhibit many useful properties, lack of good compatibility between the polyamide and ABS prevents the realization of the full potential properties of such resin blends.
As used herein, the term compatibility refers to the miscibility of two polymers. A truly compatible mixture will form an homogeneous composition which does not separate into its component phases, while incompatible polymers combine to form heterogeneous mixtures having separate phases comprising each of the components. Extrusion or injection molding of incompatible mixtures results in layered extrudates and in molded articles comprising layers of the component polymers. For truly incompatible mixtures, the layers adhere poorly and are easily separated. Where the components have marginal compatibility, the resulting layers will adhere and have a somewhat laminate character with good impact resistance and physical properties in the flow direction. As with most laminar structures, however, properties in the transverse direction are poor, and when the parts are twisted or flexed, the layers tend to delaminate and break.
Methods for preventing phase separation and delamination in mixtures of dissimilar resins have included the forming of crosslinks between the two components by peroxide or radiation techniques, thus physically tying the components together by way of covalent bonds, and the including of a third polymeric material in the form of a block or graft polymer having segments separately compatible or miscible with each of the components which thus acts as a macromolecular surfactant to increase the compatibility of otherwise nonmiscible polymers, as taught in U.S. Pat. No. 3,485,777.
More recently, in U.S. Pat. No. 3,668,274, there was disclosed a method for preparing alloys of nylon resins and a multistage modifier resin consisting of a graft copolymer having an amine-reactive moiety copolymerized in graft phase. The amine-reactive moiety, such as a copolymerizable carboxylic acid, reacts with the amine end-groups of the nylon component to form covalent bonds with the result that the nylon and graft copolymer components are chemically bound together. These alloys, in effect macro graft copolymers having polyamide chains bound to the side chains of the multistage modifier resin, do not exhibit phase separation during processing.
The methods heretofore employed for combining styrenic resins and polyamides have enjoyed some success. However, these processes which rely upon the forming of covalent bonds between polymer chains result in substantial increases in molecular weight in the final composition and have the potential for producing cross-linked resins. As will be understood by those skilled in the art, increases in molecular weight and cross-linking can greatly affect processability by lowering melt flow and otherwise effecting an undesirable decrease in tractability of the composition. Where the carboxylic acid-modified styrenic resin component is employed as an impact modifier for polyamides and forms only a minor portion of the final composition, as taught in U.S. Pat. No. 3,485,777, the level of cross-link formation is generally low and the tractability of the compositions is usually not much affected. However, where the styrenic resin components forms a substantial or major proportion of the composition, it will be apparent that cross-link formation and molecular weight increases by way of covalent bond formation can present a major problem by substantially decreasing the processability of the composition and possibly rendering it intractable. A method for providing alloys of styrenic resins and polyamides which reduces phase separation and delamination without significant covalent bond formation and cross-linking would thus lead to improved polyamide-styrenic resin compositions without sacrificing processability and tractability.